Method for autonomous self-blanking by radiation portal monitors to minimize the interference from pulsed X-rays radiation

ABSTRACT

A system and associated method for isolating intended radiation signals for determining target characteristics includes multiple detectors for detecting radiation signals having varying energies, delay modules for delaying the detected radiation signals and a discriminator associated with the multiple detectors for determining if detected radiation signals include unintended radiation signals, e.g., x-rays, and provided blanking signals to a switch in order to effectively remove the unintended radiation signals from the data that is presented to the processor for determining target characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 14/487,180, titled “Method for AutonomousSelf-Blanking by Radiation Portal Monitors to Minimize the InterferenceFrom Pulsed X-Rays Radiation,” filed Sep. 16, 2014, which is acontinuation application of U.S. patent application Ser. No. 13/672,379now U.S. Pat. No. 8,859,981, titled “Method for Autonomous Self-Blankingby Radiation Portal Monitors to Minimize the Interference From PulsedX-Rays Radiation,” filed Nov. 8, 2012, both of which are incorporatedherein by reference in their entirety.

BACKGROUND ART

Field of Embodiments

This invention relates to the field of radiation detection systems forscanning of persons, luggage, parcels, vehicles and containers for thepresence of illegal nuclear and radioactive materials. Specifically,this invention is related to methods and techniques of minimizing orremoving the unintended interference from pulsed X-ray radiationgenerated by high-energy radiography systems, operating in proximity ofpassive radiation detection systems.

Summary of Related Art

Currently, second-generation high-energy radiography systems are beingmore frequently deployed alongside passive radiation detection systems,in order to interdict illegal transportation of contraband and otherdangerous goods including nuclear and radioactive materials. It is wellknown that X-rays from nearby pulsed high-energy imaging systems caninterfere with the radiation portal monitors (RPM) used to performpassive radiation monitoring. Such interference results in an increasedcount rate measured by the RPM which could lead to, for example, falsealarms on vehicles with no radioactive materials present, when theadditional counts increase RPM's measured signal above the set alarmthreshold. Increased count rate could also raise the measured backgroundabove the actual background level when the X-ray interference occursduring RPM background acquisition, leading to a loss of sensitivity anddecreased minimum detectable activity. Second order effects of the X-rayinterference may also include: short-term gain shift, spectrumdistortion, degraded energy resolution, incomplete pile-up rejection anddecreased live time, all with negative effects on the RPM's detectionperformance.

Various solutions have been adopted in order to mitigate the effects ofpulsed X-rays radiation from pulsed X-rays radiography systems on thenearby radiation portal monitors.

A first class of such solutions is using distance, time or shielding inorder to minimize the X-rays detected by the RPM. For example, placingthe radiography system 300-500 feet away from all the RPMs operatingwithin one venue, could reduce the X-rays to a level that allows eachRPM to operate virtually free of X-ray interference. The same effectcould be achieved by surrounding radiography system with adequateshielding walls or imposing an exclusion mechanism between the timeintervals when RPMs are operating and the times when radiographysystem(s) are allowed to operate. All three approaches described above,used alone or in combination, are simple, and, in some isolated cases,cost-effective ways to address the X-rays interference. They do not relyon any particular characteristics of radiography system and RPM.Unfortunately, practical constraints imposed by limited real-estatecombined with prohibitive high cost of shielding walls, high volume oftraffic, and very large number of RPMs and radiography system that haveto operate in close proximity, reduce the number of sites where thesesolutions can be applied, to a very few isolated cases, while the vastmajority remains with X-ray interference problem unsolved.

Another widely adopted class of solutions, called hardware or predictiveblanking uses a modified RPM such that it is able to discard its inputsignals during a small time interval (typical less than 20 μs) while theX-rays pulse is generated by the radiography system. As X-rays pulserepetition rate ranges typically from 100 to 500 Hz, RPM will beblanked, in the worst case 20 μs every 2 ms, resulting in an equivalent1% dead time. This additional dead time imposed by blanking, being lessthan 1% should, in theory, have negligible effect on the RPM's detectionperformance. The method requires a hardware synchronization signalgenerated by the radiography system. This logic signal known as blankingsync, becomes active a known time interval before the X-rays pulse isgenerated. While the time delay between blanking sync and X-ray pulse isapproximately constant for a given system and operating mode, it has awide range of values for different manufacturers and/or operating modes.RPM receives blanking sync from the X-ray imaging system, and generatesan internal detector gating signal called blanking pulse. The blankingpulse is delayed with respect of the external blanking sync by a setamount known as blanking delay, and it is active for a set time intervalknown as blanking time. The values for blanking delay and blanking timeare set such that the X-ray pulse falls always inside the blanking pulsewith sufficient margins on both left and right sides. These tolerancesare required to compensate for inherent timing jitter present in thesignals involved from both participating systems: X-ray imaging and RPM.Blanking pulse is then distributed to each gamma and each neutrondetector within the same RPM. For some implementations of this method,blanking delay and blanking time may have different values for eachindividual gamma and neutron detector within the same RPM. The RPMimplementing this method is capable of disabling its gamma and neutrondetectors while internal blanking pulse is active with the end effect ofremoving all pulses while the X-rays are being generated. Hardwareblanking solution works relatively well when there is a single pulsedX-ray source, operating at a single energy with only one RPM needed tobe blanked. The major disadvantage of this method is related to therequirement that there is a wired physical connection between the X-rayimaging system and RPM, therefore making it practically impossible toimplement in the case of mobile X-rays imagining systems or when theblanking sync has to be distributed to a large number of RPMs. Thesolution becomes extremely difficult to implement when there aremultiple radiography systems with different operating modes, (singleenergy, dual energy e.g., interleaved higher and lower energy pulses),and mobile systems, operating in close proximity of a large number ofRPM systems.

There is a need in the art for an X-rays interference solution thatallows for multiple and/or mobile radiography systems, including dualenergy mode systems, operating in proximity to multiple radiation portalmonitors that eliminates pulsed X-rays interference and renders theoperation and specifically detection and nuclide classificationperformance, virtually unaffected by the pulsed X-rays generated byhigh-energy imaging systems.

SUMMARY OF THE EMBODIMENTS

In a first embodiment described herein, a method for isolating intendedradiation signals for determining target characteristics includes:detecting by a first detector photons having first energies in a firstrange and generating first radiation signals in accordance therewith;receiving by a blanking delay module each of the first radiation signalsfrom the first detector and applying a first predetermined time delaythereto; receiving by a first blanking switch each of the delayed firstradiation signals; receiving by a discriminator each of the firstradiation signals from the first detector and determining if it is anunintended radiation signal and, (i) if so determined, signaling thefirst blanking switch to blank the first delayed radiation signal inaccordance with a first blanking time to create first delayed andblanked radiation signal; (ii) if not determined, signaling the firstblanking switch not to blank the first delayed radiation signal; andreceiving by a processing component via the first blanking switch one ofthe first delayed radiation signal or the first delayed and blankedradiation signal in order to determine one or more characteristics ofthe target.

In a second embodiment described herein, a method for isolating intendedradiation signals for determining target characteristics includes:detecting by a first detector first pulses having first energies in afirst range and generating first radiation signals in accordancetherewith; receiving by a first blanking delay module each of the firstradiation signals from the first detector and applying a firstpredetermined time delay thereto; receiving by a first blanking switcheach of the delayed first radiation signals; detecting by a seconddetector second pulses having second energies in a second range andgenerating second radiation signals in accordance therewith; receivingby a second blanking delay module each of the second radiation signalsand applying a second predetermined time delay thereto; receiving by asecond blanking switch each of the delayed second radiation signals;receiving by a discriminator each of the first radiation signals fromthe first detector and determining if it is an unintended radiationsignal and, (i) if so determined, signaling the first blanking switchand the second blanking switch to blank the first and second delayedradiation signals in accordance with a first and a second blanking timeto create first and second delayed and blanked radiation signals; (ii)if not determined, signaling the first blanking switch and the secondblanking switch not to blank the first and second delayed radiationsignals; receiving by a processing component via the first blankingswitch one of the first delayed radiation signal or the first delayedand blanked radiation signal in order to determine one or morecharacteristics of the target; and receiving by a second processingcomponent via the second blanking switch one of the second delayedradiation signal or the second delayed and blanked radiation signal inorder to determine one or more characteristics of the target.

BRIEF DESCRIPTION OF THE FIGURES

The following figures exemplify the embodiments described herein and areintended to be reviewed in combination with the detailed descriptionsprovided below.

FIG. 1 illustrates exemplary signals involved in implementation ofhardware (predictive) blanking method in accordance with the descriptionprovided in the summary of the related art;

FIG. 2a illustrates an exemplary self-blanking method in accordance withan embodiment described herein;

FIG. 2b illustrates an exemplary blanking signal diagram in accordancewith an embodiment described herein;

FIGS. 3a and 3b illustrate exemplary detection configurations which maymake use of an embodiment described herein;

FIGS. 4a and 4b illustrate exemplary pulse attributes (amplitude, shapeand duration) that may be utilized to identify and blank an unintendedpulse in accordance with an embodiment described herein; and

FIGS. 5a and 5b illustrate exemplary pulse attributes for intendedpulses that may be compared with details from 4 a and 4 b to identifyand blank an unintended pulse in accordance with an embodiment describedherein.

DETAILED DESCRIPTION

The embodiments described herein are directed to a self-blanking methodwherein a radiation portal monitor (RPM) detects X-ray pulses based onone or more criteria, e.g., pulse shape analysis, and removes theidentified X-ray pulses from the measured data on the fly. The RPMdiscriminates X-rays coming from nearby pulsed X-ray imagining systemsfrom gamma rays due to natural background and/or any radioactivematerials that may be present in/or transiting the area surrounding theRPM. The method does not require synchronization or any other type ofsignals from X-ray imaging systems, or any prior knowledge of theoperating parameters of the X-ray imaging systems, such as: operatingenergy, X-ray pulse duration, single/dual energy interleaved, or othersimilar parameters. The self-blanking method eliminates pulsed X-rayinterference from one or multiple imaging systems, with fixed locationsand/or mobile systems, without the need of run-time parametersadjustment.

More particularly, the embodiments described herein use one or more of:amplitude difference between pulse X-ray sources and measured data,X-ray pulse duration and X-ray pulse shape characterization in order toisolate measured data, i.e., data representative of background gammarays and/or radioactive materials within a target passing by the RPM.

FIG. 2a depicts a schematic example of a radiation detection system inaccordance with an exemplary embodiment, in conjunction with FIG. 2bshowing the timing diagram of most significant signals present in FIG.2a . A gamma detector 1 generates an output signal 19, in response tophotons incident to the detector that may include, in addition tointended photons, unintended X-ray photons from X-ray imaging system(s)operating nearby. The gamma detector output signal 19 is then applied tothe input of an X-ray pulse discriminator module 5 and a gamma delaymodule 3. The gamma delay module 3, delays the signal 19 by a fixeddelay set to a value, in the range of 1 to 100 μs, given by a gammablanking delay parameter 15, producing at its output a gamma detectordelayed signal 20 that is in every respect identical with gamma detectoroutput signal 19 but delayed in time by a value equal to the gammablanking delay parameter 15.

On a parallel path, a neutron detector 2 generates an output signal 22,in response to neutrons and high-energy photons incident to the neutrondetector, that may include, in addition to intended radiation fromneutrons, unintended X-ray photons from X-ray imaging system(s)operating nearby. It should be noted that while the followingdescription makes reference to one neutron detection module—denoted asN1 in FIG. 2a —the embodiment of the invention has no limitations of thenumber of neutron detector modules that can be added. As an example,FIG. 2a illustrates the case when the number of neutron detector modulesused is three, denoted as N1, N2, and N3. The neutron detector outputsignal 22 is then applied to the input of a neutron delay module 4. Theneutron delay module 4, delays the signal 22 by a fixed delay set to avalue, in the range of 1 to 100 μs, given by a neutron blanking delayparameter 17, producing at its output a neutron detector delayed signal23 that is in every respect identical with neutron detector outputsignal 22 but delayed in time by a value equal to the neutron blankingdelay parameter 17.

The X-ray pulse discriminator module 5 is capable of identifying X-raypulses (also referred to herein as unintended pulses) from regulargamma-ray pulses (also referred to herein as intended pulses), based oncharacteristic attributes of the X-ray pulse such as: amplitude, widthand shape. Specifically, one exemplary implementation of the X-ray pulsediscriminator module 5 consists of a first level comparator thatgenerates a first logic signal that is active when the signal 19 ishigher than a set amplitude threshold (typically 66% of the signal 19'sfull scale), a first-order differentiator that differentiates the signal19, followed by a second level comparator that generates a second logicsignal that is active when the output of the first-order differentiatoris less than a set threshold (typically 5% of the signal 19's fullscale), and a time interval comparator that generates a third logicsignal that is active when the duration of the second logic signal islonger than a set value (typically 0.5-1.0 μs). The first, the second,and the third logic signals are combined in a AND logic gate thatgenerates a logic signal IsXPulse 6, that is active when all three logicinput signals are active.

In one exemplary embodiment, the X-ray pulse discriminator module 5generates the logic signal IsXPulse 6, for every X-ray pulse that hasbeen identified as including pre-established identifying characteristicsfor one or more of amplitude, width and shape, and preferably, all threecriteria. The time when this decision takes place is marked by therising edge of the logic signal 6. The logic signal 6 is thentransferred to the input of a gamma blanking control module 7 and aneutron blanking control module 8 for further processing. Gamma blankingcontrol module 7 generates a gamma blanking logic signal 13 that has aduration controlled gamma blanking delay 15 and gamma blanking time 16,in the range of 1 to 100 μs. Gamma blanking logic signal 13 is activestarting from the rising edge of the logic signal IsXPulse 6 and has aduration equal with the sum of gamma blanking delay 15 and gammablanking time 16. Gamma blanking logic signal 13 controls the operationof a gamma blanking switch 9 such that the switch 9 is set in position“A” when gamma blanking signal 13 is inactive and in position “B” whenthe blanking signal 13 is active. Gamma blanking switch 9 generates asignal equal with gamma detector delayed signal 20 at all times whengamma blanking signal 13 is inactive and a signal ground (zero) whengamma blanking signal 13 is active, with the end effect of removing(blanking) from the original gamma detector signal 19, all pulsespresent in the time interval equal with gamma blanking delay 15 prior tothe rising edge of the logic signal IsXPulse 6, and all pulses presentin the time interval equal with gamma blanking time 16 after the risingedge of the signal IsXPulse 6. As a consequence of its operation, gammablanking switch 9 generates a gamma detector signal delayed and blanked21, that has been “purged” of all pulses occurring in a time intervalequal with the sum of gamma blanking delay and gamma blanking time(gamma blanking delay before, and gamma blanking time after the X-raypulse has been detected) as all these pulses are suspected of beingcaused or affected by the X-ray interference.

Gamma detector signal delayed and blanked 21 is then applied to input ofa gamma spectrometer 11 that performs normal signal processing andgenerates the measured spectrum free of X-ray interference. Optionally,gamma blanking signal 13 could be input into the gamma spectrometer 11providing information related with the dead time added by blanking Gammaspectrometer 11 could use this information to perform a dead timecorrection if necessary and/or desired. Alternatively gamma spectrometer11 could use the information provided by gamma blanking signal 13 tomaintain a correct live time measurement and pass this value along withthe measured spectrum.

Similarly, neutron blanking control module 8 generates a neutronblanking logic signal 14 that has a duration controlled neutron blankingdelay 17 and neutron blanking time 18. Neutron blanking logic signal 14is active starting from the rising edge of the logic signal IsXPulse 6and has a duration equal with the sum of neutron blanking delay 17 andneutron blanking time 18, in the range of 1 to 100 μs. Neutron blankinglogic signal 14 controls the operation of a neutron blanking switch 10such that the switch 10 is set in position “A” when neutron blankingsignal 14 is inactive and in position “B” when the blanking signal 14 isactive. Neutron blanking switch 10 generates a signal equal with neutrondetector delayed signal 23 at all times when neutron blanking signal 14is inactive and a signal ground (zero) when neutron blanking signal 14is active, with the end effect of removing (blanking) from the originalneutron detector signal 22, of all pulses present in the time intervalequal with neutron blanking delay 17 prior of the rising edge of thelogic signal IsXPulse 6, and all pulses present in the time intervalequal with neutron blanking time 18 after the rising edge of the signalIsXPulse 6. As a consequence of its operation, neutron blanking switch10 generates a neutron detector signal delayed and blanked 24, that hasbeen “purged” of all pulses occurring in a time interval equal with thesum of neutron blanking delay and neutron blanking time (neutronblanking delay before, and neutron blanking time after the X-ray pulsehas been detected) as all these pulses are suspected of being caused oraffected by the X-ray interference.

Neutron detector signal delayed and blanked 24 is then applied to inputof a neutron spectrometer 12 that performs normal signal processing andgenerates a measured spectrum free of X-ray interference. Optionally,neutron blanking signal 14 could be input into the spectrometer 12providing information related with the dead time added by blanking. Thespectrometer 12 could use this information to perform a dead timecorrection if necessary and/or desired. Alternatively the spectrometer12 could use the information provided by blanking signal 14 to maintaina correct live time measurement and pass this value along with themeasured spectrum.

It should be noted that the rising edge of the logic signal IsXPulse 6occurs at the time when the X-ray pulse has completely returned to thebaseline value. As a consequence, the information that an X-ray pulsehas been detected is available at the time when the X-ray pulse isalready part of the data stream to be processed by both gamma andneutron processing modules 11 and 12. In the absence of the delaymodules 3 and 4, the X-ray pulse would be already processed and recordedin the measured data by both processing modules 11 and 12, by the timethe logic signal IsXPulse 6 becomes active. Moreover, the delay modules3 and 4, become strictly necessary when the pulsed X-ray source used bythe radiography system is of the type known as “betatron”. It is wellknown that this type of X-ray source is prone to produce “early X-rays”before the main, full energy X-ray pulse. Even if “early X-rays” are notdetected by the X-ray pulse discriminator module 5, they are completelyremoved by blanking as they occur within blanking delay time interval,prior to the main X-ray pulse.

Referring to FIG. 2b , diagrams A-E illustrate the various blankingsignals versus time in accordance with the embodiment described withreference to FIG. 2a . While FIG. 2b exemplifies the delay and blankingwith reference to the gamma detector, one skilled in the art recognizesthat the same concepts apply in the parallel paths for additionaldetectors (e.g., neutron detector modules N1, N2, N3).

More particularly, diagram A shows an exemplary gamma detector outputsignal (19 in FIG. 2a ), containing an unintended X-ray pulse precededby several “early X-ray pulses”. Diagram B shows the IsXPulse 6 fromFIG. 2a which drives blanking signal 13 with blanking time T_(B)(diagram C). Next, diagram D shows the delay gamma output signal 20 fromdiagram A after it has been delayed T_(D) by the delay module (3 in FIG.2a .) and finally diagram E depicts the delayed and blanked gammadetector signal 21 after all unintended pulses (X-ray main pulse andpotential “early X-rays”) have been removed from the original detectorsignal.

The types of individual detectors referenced in the exemplary system inFIG. 2a may be used to scan for and detect special nuclear material(SNM) which are used to create atomic weapons, radiological dispersaldevices (RDD or “dirty bombs”), and other types of radioactivematerials. While the individual systems, and in particular thedetectors, are intended to receive and process a specific type ofradiation, the individual systems may detect stray signals from othersystems operating in proximity thereto. This results in inaccurateperformance of the first system, e.g., obscured results, false alarms,etc. A more detailed description of the various types of scanningsystems that may be utilized in conjunction with the process describedherein is found in one or more of the following United States patentswhich are incorporated herein by reference in their entirety: U.S. Pat.Nos. 7,045,787; 6,255,654; 6,507,025; 7,408,160; 6,552,346; 7,365,332;7,368,717; 7,388,205; 7,335,887; 7,039,159; 7,215,738; 7,352,844;7,596,275; 7,453,987; 7,166,844; 7,388,209; 7,430,479; 7,742,568;7,957,506 and 8,116,431. Additionally, commonly owned U.S. patentapplication Ser. No. 11/033,552 entitled System and Method for Measuringand Analyzing Target Emissions is also incorporated herein by referencein its entirety.

Additionally, while one or more neutron detectors are discussed withreference to the embodiment in FIG. 2a , the self-blanking method may beutilized to blank only the gamma detector. Further, one skilled in theart recognizes that the functionality represented and described withreference to FIG. 2a may be implemented through software, hardware or acombination thereof.

In FIG. 3a , a more specific example of the dual radiation detectionsystem in FIG. 2a includes two different detection systems 110 and 120Aoperating in proximity to one another. The passive system 110, is usedto scan all or some portion of the vehicle 112 as it drives betweendetector panels 114. Exemplary embodiments of system 110 include ScienceApplications International Corporation's EXPLORANIUM AT-980, AT-900,ST-20 and SRM-910 Radiation Portal Monitors. Similarly, system 120A isrepresentative of an active scanning system, wherein a x-ray source 122generates x-rays, e.g., at approximately 5-7 MeV and 8-10 MeV of linacand approximately 5-7.5 MeV range for betatron source, which passthrough vehicle 116 and are received at detector 124. The system 120A inFIG. 3a is intended to represent a mobile or movable system, such asthat shown in FIG. 3A of U.S. Pat. No. 7,408,160, which is incorporatedherein by reference and which is physically embodied in commerciallyavailable products such as Science Applications InternationalCorporation's VACIS M6500 Mobile Imaging System. It is common formultiple scanning systems such as those described herein to besimultaneously operated within proximity to one another in order toefficiently and effectively monitor the contents of cargo at bordercrossings, ports or other checkpoints.

In FIG. 3b , an alternative configuration shows two different detectorsystems 110 and 120B, wherein the features and operation of system 110are identical to those described with reference to FIG. 3a and thefeatures and operation of system 120B are identical to those describedwith reference to 120A except 120B is a stationary system. As shown,systems 110 and 120B may simultaneously operate on different vehicles,but as exemplified by ray R3, unintended radiation from source 122 maybe detected by one or more detectors 114. The embodiments foridentification and blanking as described herein may be utilized. As afurther alternative, it may be possible to operate the source 122 ofsystems 120A and 120B in a dual mode configuration, wherein operation ateither 5-6 MeV or 8-10 MeV is selectable. This includes an interlacedoperation wherein the two energies are alternately used as described inU.S. Pat. No. 7,453,987 which is incorporate herein by reference.

Importantly, the detectors 114 of system 110 may include different typesof individual detectors for detecting emissions having differentenergies. As described in commonly owned U.S. patent application Ser.No. 11/033,552 which is incorporated herein by reference and entitledSystem and Method for Measuring and Analyzing Target Emissions, FIG. 1of this application illustrates a detector stand that includes bothneutron and gamma ray detectors. Importantly, gamma ray detectors aresensitive to source generator x-rays such as those generated by source122 of system 120A. As is known in the art, shielding and focusingtechnologies are utilized in order to confine the emitted x-rayradiation to a designated scanning area, but these technologies are notperfect. There is still some amount of stray radiation that escapesconfinement. Since the detectors 114 are sensitive, that is, they arecalibrated to measure small amounts of radiation emitted by materials inthe passing targets, it is understandable that additional strayradiation from an x-ray source could mask gamma and/or neutron dataemitted from a target or cause a false-positive. For exemplary purposes,detectors 114 may include polyvinyl toluene (PVT) detectors which havebeen shown to be capable of detecting radiation from both gamma-ray andneutron sources as discussed in the Sandia Report entitled PVT-NG SensorFinal Report printed January 2012 which is incorporated herein byreference in its entirety. As discussed above, predictive blanking hasbeen implemented in the prior art and involves an external device fordetecting the radiation from the sources 122 and essentially telling thedetectors 114 when to “blank.” The embodiments described herein do notuse an external device to provide the blanking signal. Instead, thesystem 110 is modified per the description in FIG. 2a to include aself-blanking feature which implements one or more signal processingmethodologies to effectively remove the unintended x-ray radiation fromthe detected data. Assuming the detected data is (intendedradiation+unintended radiation), wherein the intended radiation containsinformation regarding the contents of the target being scanned bydetectors 114 and the unintended radiation does not, the processesdescribed herein work to process the detected data to identify andremove the unintended radiation per the process described in accordancewith the schematic of FIG. 2 a.

As discussed above in the example provided with reference to FIG. 2a ,the x-ray pulse discriminator identifies various pulse characteristicsto determine if the pulse is an unintended x-ray pulse. The signalprocessing methodologies implemented by the discriminator foridentifying the characteristics utilize anticipated attributes of theunintended impinging radiation in order to selectively remove (or blank)the unintended radiation signals from the detected signals. Referring toFIG. 4a , it is anticipated that full energy x-ray pulses at, e.g., 6.5MeV and 9.0 MeV, that are incident on detectors 114 will have amplitudesthat are higher than a set threshold. Accordingly, referring back toFIG. 3a for situations wherein some portion R1 of the source 122generated energy impinges directly on a detector 114, it is expectedthat the amplitude of the resulting detected signal will exceed apredetermined threshold and thus be easily identifiable and removed.More particularly, FIG. 4a illustrates an exemplary near field directx-ray signal and shows that the resulting signal has an amplitude thatis essentially off the scale and significantly above an intendedradiation threshold. Similarly, even a far field x-ray signal that isfacing a detector 114 has an amplitude that is significantly above anintended radiation threshold (see FIG. 4b ). These amplitudes for theunintended x-ray signals as illustrated in FIGS. 4a and 4b can becompared to the amplitudes expected from an intended radiation signals,e.g., gamma ray signals, shown in FIGS. 5a (high energy) and 5 b (lowenergy) which are significantly lower.

It is also anticipated that scattered x-ray pulses will likely generatepile-up events when detected by detectors 114 even when they have losttheir primary energy, (see FIG. 4b ). The scattered x-ray pulsesrepresent source x-rays that have taken an indirect path to the detector114. More specifically, scattered x-rays have encountered a structure,thus changing the attributes of the scattered x-ray signal in one ormore ways from the original, un-scattered signal emitted from the source122. Accordingly, referring back to FIG. 3a for situations wherein someportion R2 of the source 122 generated energy encounters a structure,e.g., truck cab 112, and then impinges on a detector 114, it is expectedthat the scattered x-ray signal will generate pile-up events and thatsuch pile-up events will be identified and the corresponding signalsremoved from the detected data. Pulse pile-up happens when pulsesarrives closer in time than the pulse resolution time for the detector114. When pile-up occurs, the detector returns inaccurate pulse heightinformation. For example, the separate pulse may be incorrectly viewedas a single pulse when the pulses are very close in time and amplitudewill be calculated by adding the two amplitudes, e.g., peak pile-up. Thedetector could also record the overlapping signals as two separateevents, but calculate incorrect pulse amplitude due to the overlap,e.g., tail pile-up. Since the intended radiation is not expected toproduce pile-up since the detectors 114 are tailored to avoid such anphenomenon in the range of the intended radiation, pile-up events can beattributed to unintended radiation and thus removed.

While various embodiments of the invention have been described andillustrated, one skilled in the art recognizes that there are variationsto these embodiments and implementations thereof that are and would beconsidered to be within the scope of those embodiments.

The invention claimed is:
 1. A self-blanking radiation portal monitorfor discriminating between gamma-ray target and neutron target radiationindicative of target contents and x-ray non-target radiation, theself-blanking radiation portal monitor comprising: at least a firstdetector for receiving input radiation including at least one ofgamma-ray target radiation and x-ray non-target radiation and generatingoutput signals indicative of the input radiation; and a blanking circuitfor processing the output signals to blank out the x-ray non-targetradiation indicated therein and provide only gamma-ray target radiationsignals, wherein the gamma-ray target radiation signals are used toimage the target contents and further wherein the blanking circuitcomprises: a blanking delay module for receiving each of the outputsignals from the at least a first detector and applying a firstpredetermined time delay thereto; a first blanking switch for receivingeach of the delayed output signals; a discriminator for receiving eachof the output signals from the first detector and determining if itincludes x-ray non-target radiation and signaling the first blankingswitch in accordance with the determination.
 2. The self-blankingradiation portal monitor according to claim 1, wherein determining bythe discriminator if an output signal includes x-ray non-targetradiation includes determining if a predetermined criteria for at leastone of amplitude, width and shape of the output signal is met.
 3. Theself-blanking radiation portal monitor of claim 1, wherein the at leasta first detector is a polyvinyl toluene (PVT) detector.
 4. Theself-blanking radiation portal monitor according to claim 1, wherein thediscriminator further includes at least one comparator and at least onedifferentiator.
 5. The self-blanking radiation portal monitor accordingto claim 1, further comprising: a second detector for receiving neutronradiation and generating a neutron output signal; and a second blankingdelay module for receiving each of the neutron output signal from thesecond detector and applying the first predetermined time delay thereto;a second blanking switch for receiving the delayed neutron output signaland receiving a blanking signal from the discriminator, wherein theneutron output signal is used to determine the target contents.
 6. Ablanking circuit for processing detector output signals indicatingcontents of a target, comprising: a first blanking delay module forreceiving first output signals from at least a first detector andapplying a first predetermined time delay thereto; a first blankingswitch for receiving each of the delayed output signals; a discriminatorfor receiving each of the output signals from the first detector anddetermining if it includes x-ray non-target radiation and signaling thefirst blanking switch in accordance with the determination to blank outthe x-ray non-target radiation indicated therein and provide onlygamma-ray target radiation signals, wherein the gamma-ray targetradiation signals are used to image the target contents.
 7. The blankingcircuit according to claim 6, wherein the discriminator further includesat least one comparator and at least one differentiator for determiningif a predetermined criteria for at least one of amplitude, width andshape of the detected output signal is met.
 8. The blanking circuit ofclaim 6, further comprising; a second blanking delay module forreceiving second output signal from a second detector and applying thefirst predetermined time delay thereto; and a second blanking switch forreceiving the delayed second output signals and receiving a blankingsignal from the discriminator, wherein the delayed second output signalis used to determine the target contents.
 9. The blanking circuit ofclaim 6, wherein the at least a first detector is a polyvinyl toluene(PVT) detector.
 10. The blanking circuit of claim 6, wherein the seconddetector is a neutron detector.
 11. A system for determining thecontents of a target comprising: at least one neutron radiationdetector; at least one radiation source and at least one gamma radiationdetector; and at least one blanking circuit for isolating neutronradiation from the target and gamma radiation from the target fromnon-target radiation wherein the blanking circuit comprises: a firstblanking delay module for receiving first output signals from the gammaradiation detector and applying a first predetermined time delaythereto; a first blanking switch for receiving each of the delayedoutput signals; a discriminator for receiving each of the output signalsfrom the gamma radiation detector and determining if it includesnon-target radiation and signaling the first blanking switch inaccordance with the determination to blank out the non-target radiationindicated therein and provide only gamma radiation signals.
 12. Thesystem of claim 11, wherein the discriminator further includes at leastone comparator and at least one differentiator for determining if apredetermined criteria for at least one of amplitude, width and shape ofthe detected output signal is met.
 13. The system of claim 11, furthercomprising; a second blanking delay module for receiving second outputsignals from the neutron detector and applying the first predeterminedtime delay thereto; and a second blanking switch for receiving thedelayed second output signals and receiving a blanking signal from thediscriminator, wherein the delayed second output signal is used todetermine the target contents.
 14. The blanking circuit of claim 11,wherein the gamma radiation detector is a polyvinyl toluene (PVT)detector.
 15. The system of claim 11, wherein the at least one radiationsource and at least one gamma radiation detector are mobile duringoperation and the at least one neutron detector is stationary duringoperation.